Ida's moon: a sharper view.

Last spring, images radioed by the Galileo spacecraft from a 6-hour encounter with the asteroid 243 Ida revealed a tiny satellite orbiting this rocky body- the first moon of an asteroid ever photographed (SN: 4/2/94, p.214). NASA recently released Galileo's sharpest view of the moon, taken a year ago and radioed in June. The image of the kilometer-size, egg-shaped moon resolves features as small as 50 meters across.

The picture shows more than 12 craters that exceed 80 m in diameter, indicating that the tiny body has taken quite a battering from solar system debris. In fact, this suggests that the moon can't be more than a few hundred million years old, says Michael J.S. Belton of the Kitt Peak National Observatory in Tucson. An older object this small probably wouldn't have survived the additional impacts, he adds.

Belton suggests that the direct ancestor of the moon is not the same one that fragmented to create Ida. But both the asteroid and its satellite ultimately stem from the breakup of the much larger body that formed the Koronis asteroid family he says. Infrared spectra taken by Galileo also suggest that Ida's moon isn't just a chip gouged from Ida. These data show that the surface abundance of the minerals pyroxene and olivine on the moon differs from that on Ida, says Robert W. Carlson of JPL.